It's a rite of passage for young men, a chance to test their bravery in the streets of Pamplona, Spain.
On Saturday, half a dozen specially bred and aggressive bulls raced from their pen along a winding 875-metre route to the city's bull ring.
For the men and the few women who take part, it's a risky undertaking.
The "running of the bulls" lasts for a week. Every year some people are injured and even killed.
On Saturday, five people were taken to hospital, although none of them were directly gored by a bull.
Two suffered arm injuries after falling, two others bruising to the head and one a torso injury.
Then later in the day, to the outrage of animal rights supporters, the bulls face death at the hands of matadors in the city's bull-ring.